Vector network analyzers (VNAs) include a signal source to stimulate a device under test (DUT) and one or more tuned receivers to measure responses of the DUT to the stimuli provided by the source. Traditionally, VNAs measure transmission and/or reflection responses of the DUTs, with the receivers tuned to perform measurements at the same frequency as the source. Modern VNAs with offset tuning capabilities enable the frequencies of the source and the receivers to be set independently. Thus, the VNA can stimulate the DUT at one or more fundamental frequencies while measuring the response of the DUT at frequencies that are different from the fundamental frequencies, such as harmonic frequencies, subharmonic frequencies, frequencies of intermodulation (IM) distortion products, or other distortion signals. However, since the source supplying the stimuli to the DUT at the fundamental frequencies typically provides unwanted distortion signals due to limitations in spectral purity of the source, measuring the distortion generated by the DUT using the VNA involves accounting for the distortion signals that are attributable to the source.
Kepetanic et al. (U.S. Pat. Nos. 6,416,945 B1; 6,396,287 B1; 6,292,000 B1) disclose processes for determining relative harmonic levels of a DUT using a VNA. Kepetanic et al. relies on a vector difference equation, which includes a ratio of an output harmonic level to a harmonic level from the source, to determine these relative harmonic levels. When the harmonic levels of the source are sufficiently low to be on the order of the noise floor of the source and the receivers of the VNA, the determination of the relative harmonic levels becomes susceptible to errors, because the denominator of the ratio is influenced by the noise floor of the VNA. Accordingly, there is a need for distortion measurements that accommodate for both low level and high level harmonics from a source.